In a known type of nuclear power reactor, for example, a boiling-water reactor, nuclear fuel is provided in elongated rods. The nuclear fuel is typically in the form of uranium oxide and/or plutonium oxide pellets enclosed in zircaloy tubes. A number of such fuel rods are grouped together in an open-ended tubular flow channel. The flow channel with the fuel rods positioned therein is referred to as a "fuel assembly" or "bundle." A plurality of fuel assemblies are removably positioned in the reactor core in a matrix. The reactor core formed in this manner is capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
The fuel rods in a fuel assembly are supported between upper and lower tie plates. The fuel rods are typically arrayed in parallel side by side vertical upstanding relation. The fluid coolant flows past the fuel rods in the intersticies between the vertical and parallel fuel rods. To provide proper coolant flow and preserve integrity of the fuel rods, it is important to maintain the rods in a preferred spaced relationship and to restrain them from bowing and vibrating during reactor operation.
To maintain spaced apart alignment of its fuel rods, a plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose. One type of spacer includes a plurality of generally cylindrical ferrule elements. An example of a spacer usable in a fuel assembly is that shown in U.S. Pat. No. 4,508,679, issued Apr. 2, 1989, to Matzner, et al.
As shown in Matzner, et al., one method of positioning a fuel rod within the ferrule elements of a spacer is to provide a spring member for biasing the fuel rod against rigid stops in the ferrules. The spring depicted in U.S. Pat. No. 4,508,679 is in the form of a continuous loop of generally elliptical shape. The springs are positioned in the volume of the spacer area between adjacent fuel rods.
Modern fuel bundle design typically includes fuel rods being arrayed in a square sectioned fuel bundle. The arrays originally where in a 7.times.7 matrix. This left relatively large spaces between the fuel rods. Accordingly, the problem of placing springs between the rod to maintain the rods in vertical upstanding relation presented a generally simple mechanical design problem.
Unfortunately, modern fuel bundle designs include much denser fuel rod arrays. Such arrays have gone from fuel rod matrices including 8.times.8 fuel rod arrays to 9.times.9 and 10.times.10 fuel rod arrays. This being the case, the interstitial volume (or thickness) between the fuel rods has shrunk. Although the same spring action is required for the most part from the springs acting with the spacers to maintain the fuel rods in vertical upstanding relation, the space in which such spring action can occur is vastly reduced. Previous fuel assemblies having 8.times.8 matrices fuel rods had been constructed with rod-to-rod spacings (i.e.. distances between outer circumferences of adjacent rods) of about 0.160 inches (about 4 mm). Modern fuel bundles are being designed with 9.times.9 matrices of fuel rods to have a reduced rod-to-rod spacing, such as about 0.12-0.14 inches (about 3 to about 3.5 mm). This reduction in fuel rod interstitial spacing has had severe constraints on the spring design.
It should be noted that the springs, although necessary for positioning the rods, can have certain undesirable effects. These undesirable effects include absorption of neutrons and interference with the coolant circulation. Materials from which the springs are typically formed absorb 20-100 times the number of neutrons absorbed by the spacer material. Accordingly, there is a high motive to maintain a minimum of spring material within the fuel bundle.